1. Field of the Invention
This invention generally relates to the fabrication of integrated circuits and, more particularly, to a system and method for forming polysilicon (p-Si) films from the sputter deposition of amorphous Si (a-Si) using an Argon (Ar)-Hydrogen (H2) gas mixture.
2. Description of the Related Art
Amorphous Si thin films are used in the fabrication of polycrystalline silicon thin film transistors (TFTs), which in turn are a key element in active matrix (AM) type liquid crystal displays (LCDs).
Sputtering is a well-known conventional process suited to the formation of the various Si-based, TFT layers because:
1. It is a safe and environmentally benign technique;
2. It can be used even at room temperature. Hence, it is compatible with any kind of substrate;
3. Silicon films with very low H2 content can be deposited by this method. Hence, there is no need for dehydrogenation to release excessive hydrogen. Alternatively, hydrogen can be incorporated into the film if, and when, necessary; and
4. It is a simpler and a more easily scaled method than comparable methods that rely upon chemistry.
For improved electrical performance it is desirable that the deposited amorphous silicon film be converted, typically by annealing, into a microcrystalline or polycrystalline Si film. There are many processes known in the art to form polycrystalline silicon (polysilicon) from amorphous silicon.
It is known to use a large amount of hydrogen in the Ar gas in the sputtering deposition process for applications such as deposition of hydrogenated a-Si:H films used in amorphous silicon solar cells or amorphous silicon LCDs. In these films, H2 termination of Si dangling bonds is necessary to achieve desired film characteristics. However, the use of hydrogen in the sputtering deposition process is not conventionally known to improve the electrical characteristics of the polycrystalline silicon film formed by this method.
FIGS. 1a through 1e are partial cross-sectional diagrams illustrating the fabrication of a conventional top-gate TFT structure (prior art). Poly-Si (polycrystalline-Si) TFTs are made by a plurality of processes. In the majority of polycrystalline silicon TFT LCD applications, the so-called top-gate, polycrystalline silicon TFT structure is used. Typically, Plasma-Enhanced Chemical Vapor Deposition (PE-CVD) or Low-Pressure CVD (LPCVD) is used to deposit the amorphous silicon precursor. However, there are several advantages in using physical vapor deposition (PVD) or sputtering to form the silicon film. Such advantages are a reduction in process steps, since there is no need for dehydrogenation, a reduction in equipment costs, and improved process safety, since no toxic/pyrophoric gases are necessary.
In FIG. 1a a barrier layer 10 is deposited over a substrate 12. Amorphous Si 14 is deposited over barrier layer 10.
In FIG. 1b the silicon layer 14 is annealed, using an Excimer Laser for example, to form polycrystalline silicon layer 14. Then, the polycrystalline silicon layer 14 is patterned and dry etched.
In FIG. 1c a gate isolation layer 16 is formed over the polycrystalline silicon layer 14. A gate 18 is formed over gate isolation layer 16, and the source region 20 and drain region 22 are implanted with P material.
In FIG. 1d an interlayer dielectric 24 is isotropically deposited.
In FIG. 1e the interlayer dielectric 24 is selectively etched to form vias to the source/drain regions 20/22. A source contact 26 and a drain contact 28 are deposited and patterned. The present invention is concerned with the sputter deposition of the amorphous silicon used to form polycrystalline silicon layer 14 (FIG. 1a).
FIG. 2 is a partial cross-sectional diagram of a typical DC magnetron sputtering chamber (prior art). One of the key aspects of the Si-sputtering process is the xe2x80x98targetxe2x80x99 component. The target is a block of the material to be deposited, mounted on an appropriate metal backing plate, and placed opposite to the substrate where the film is to be deposited. Plasma strikes in the gap between the target and the substrate. The magnet that is scanning above the target backing plate is used to intensify the plasma and confine it in the region defined by the magnetic field. By scanning the magnet, the plasma is swept across the surface of the target, resulting in deposition of the film on the substrate opposite to the target. The plasma is generated by applying high voltage to an inert gas (typically Ar, but alternately He, Ne, Kr or mixtures) that flows in the region between the target and the substrate. For certain applications, other gases may be mixed to the sputtering gas, such as H2, O2, N2, etc., to alter the composition and/or the properties of the sputtered film.
It would be advantageous if a process existed for improving the electrical characteristics associated with an amorphous silicon film deposited by sputtering.
It would be advantageous if a process existed for improving the sheet resistance of polycrystalline silicon film, annealed from a sputter deposited amorphous silicon film.
It would be advantageous if the above-mentioned polycrystalline silicon film could be fabricated using easily controlled process steps and inert materials.
Accordingly, a method is provided for forming a polycrystalline silicon (p-Si) film in an integrated circuit (IC) fabrication process. The method comprises: sputtering amorphous silicon (a-Si) material on a substrate; supplying an Ar gas mixture including a hydrogen content of no more than 4% volume (in the gas feed); forming an amorphous silicon film incorporating hydrogen; annealing the amorphous silicon film using a rapid thermal annealing or Excimer laser process; and, forming a polycrystalline silicon film having a crystalline content in the range from 95 percent (%) to 100%, as measured by Raman Spectroscopy or equivalent measurement means, and a hydrogen content in the range from 1% atomic weight (at %) to 3 at %.
Also provided is a polysilicon (p-Si) film, such as might be used in the fabrication of a thin film transistor (TFT). The polycrystalline silicon film comprises a pre-anneal amorphous silicon (a-Si) film having a content of no more than 4% hydrogen, by atomic weight, and a post-anneal polycrystalline silicon film having a crystalline content in the range from 95% to 100%, and a hydrogen content in the range from 1 at % to 3 at %.
Additional details of the polycrystalline silicon film, polycrystalline silicon film fabrication process, and a pre-anneal film are provided below.